A manifesto pledge to require drugs companies to publish full details of clinical trials for medicines cannot be fulfilled because it would be illegal under EU law, the government has admitted.

Labour’s 2005 election manifesto promised to “require registration of all clinical trials and publication of their findings for all trials of medicinal products with a marketing authorisation in the UK”.

But the Department of Health has now admitted it would be illegal under EU regulations to do this.

As things stand, the pharmaceutical industry is encouraged to publish findings of all clinical trials online but largely regulates itself.

Critics argue the system does not do enough to ensure that patients, doctors and watchdogs, such as the National Institute for Clinical Excellence (Nice), get the information they need to assess drugs’ effectiveness.

A spokeswoman for the Department of Health said: “We planned to support the principle of mandatory registration of clinical trials but legal advice stated this would be illegal under EU law.”

She said the department was pressing the EU to make its medicines agency register public.

The government’s admission follows the publication of a review of clinical trials in the 1980s and 1990s that found that antidepressant drugs were no more effective than placebos for patients suffering from depression.

The review examined published and unpublished data from clinical trials obtained under freedom of information rules in the US and led to calls from psychiatrists and mental health campaigners for greater openness on trial results.

Nice has pledged to consider the findings when it reviews its guidance on treatments for depression this year.